Among birds such as chickens, which include varieties raised mainly for their eggs such as the White Leghorn, only the egg-producing females are useful. By contrast, among chickens raised for their meat, both the males and the females are useful, although males and females mature at different rates and it is therefore more efficient to raise them separately. In the case of both types of the above-described varieties of chickens, for these and other reasons the chicks are sexed 2–3 days after they are hatched in order to determine what sex they are.
Three methods are currently used to determine the sex of the hatchlings: (a) by hand, (b) by machine or (c) by utilizing sex-controlled inheritance (that is, sex-linked inheritance) characteristics.
All three of the above-described conventional methods involve sexing a hatched chick, so unless the egg is hatched the sex of the chick to be hatched cannot be determined. As a result, in the case of the White Leghorn chickens raised to produce eggs, for example, the time and expense of hatching eggs containing male chicks is wasted. In addition, in the case of such egg-producing varieties of chickens, those chicks found to be male are destroyed, which is undesirable from the viewpoint that life in all its forms should be respected.
If it were possible to reliably sex the chicks before they are hatched, that is, while still in the egg, the above-described wastage and dilemma could be avoided. Prior to hatching, for example, the males could be used for food or for the production of vaccines.
A method for determining the sex of a fertilized chicken egg, based on a shape of the egg, has been practiced for a long time in Kyushu or in Southeast Asia. In this method, references are established respectively for a male egg and a female egg for every parent chicken, primarily in terms of a shape of a wide side of the egg (a blunt end side having an airspace), that is, a bulge of the wide side of the egg. Then, an egg within the reference range has been found to be a female egg or a male egg.
In addition, a method for comparing shapes of blunt end portions (that is, end portions having larger diameters and roundness) of various chicken eggs has also been used. In this technique, a profile projector is used to take a blown-up profile of the laid egg for each hen, after which the eggs are allowed to hatch, the hatchlings are sexed, and the shapes of the eggs are categorized according to the sex of the resulting chick. This process is repeated over a certain period of time until a reference range is established for each hen, after which reference profiles are produced. Thereafter, the shape of a laid egg to be sexed is then compared to the reference profiles in order to determine the sex of the unhatched chick.
However, the above-described conventional chicken egg sexing technique has the following drawbacks.
(a) It is commercially impractical to get reference profiles for each hen, because a lot of chickens are bred in the poultry farming business.
(b) The technique relies on the human eye to compare the blunt end of the egg to be sexed against the reference profile established for that hen, and as such is not entirely reliable.
(c) The work of producing the reference profiles for sexing the eggs involves the above-described steps, so it is a relatively lengthy process.
(d) Eggs (especially, fertilized eggs) available in recent years having a conventional shape such that a wide side of the egg is bulged and a narrow side of the egg is narrower than the wide side have gradually decreased, whereas round eggs, or eggs whose maximum bulge portions are positioned at the center of thereof, or elongated eggs have increased. Thus, the shape of the egg is very complicated, so that it is difficult to determine the sex in regard to all of the eggs consistently according to only a part of the references of the egg shape. Therefore, it becomes difficult to determine the sex with high accuracy. In addition, the determination of the sex based upon only the bulge or roundness of the wide side is insufficient to definitely classify the eggs into males and females and leads to errors.
Therefore, in the method for determining the sex based upon the egg shape as in the past, it is difficult to perform the determination with high accuracy.